Green’s Dictionary of Slang

(orig. US black/teen) a general term of approval, attractive, excellent etc.

1997		Mad mag. Apr. 32: Boo-yaaa!!! ’Cause Hooked on Ebonics is butter!
1997–2000		Da Bomb 🌐 5: Butter: Impressive.
1998		Source Oct. 22: Redman and Keith Murray come through with some butta lyrics.
2014		C. Eble (ed.) UNC-CH Campus Sl. Spring 2014 2: BUTTER — done with skill: ‘James-Michael’s dunk was butter’ .